Recently, wind turbines have received increased attention as an environmentally safe and a relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a plurality of blades coupled to a hub forming a turbine rotor. Utility grade wind turbines (i.e. wind turbines designed to provide electrical power to a utility grid) can have large turbine rotors (e.g., seventy or more meters in diameter). Blades on these turbine rotors transform wind energy into a rotational torque or force that drives a rotor of one or more generators. The turbine rotor is supported by the tower through a set of internal bearings that include a fixed portion coupled to a rotatable portion. The set of internal bearings is subject to a plurality of loads including the weight of the turbine rotor, a moment load of the turbine rotor that is cantilevered from the set of internal bearings, symmetric and asymmetric wind loads loads, such as, horizontal and shears, yaw misalignment, and natural turbulence.
In a direct drive wind turbine generator, the generator rotor is directly coupled to the turbine rotor. The generator rotor and stator are separated by an airgap. During operation, a magnetic field generated by permanent magnets or an excited wound field mounted on the generator rotor passes through the airgap between the rotor and the stator. The passage of the magnetic field through the airgap is at least partially dependent on the uniformity of the airgap. Asymmetric and/or transient loads on the generator may be introduced through the turbine rotor from the blades. Such loads are transmitted from the turbine rotor to the wind turbine base through the set of internal bearings and may tend to deflect structural components of the generator rotor and stator in the load path such that the airgap distance is reduced and/or made non-uniform. One proposed solution includes fabricating wind turbine components from stiffer and/or stronger materials capable of withstanding the loads on the rotor. However, the size and/or weight drawbacks of stiffer and/or stronger materials and/or components make their use prohibitive. Additionally, the substantial structure needed to control the airgap would use up valuable hub-access space needed to install and service systems such as pitch-control and other devices.
Thus, what is needed is a method and system to provide a wind turbine generator having an arrangement of a rotor and a stator that provides airgap stability.